Telephone system regulatory requirements in the United States and elsewhere call for customer premise equipment (e.g., telephones, modems, facsimile machines, etc.) to provide galvanic isolation between the circuitry of the customer premise equipment (CPE) and the public switched telephone network (PSTN) if the CPE is to qualify for connection to the PSTN. Consequently, CPE customarily includes a circuit, known as a data access arrangement (DAA) circuit, which provides the required galvanic isolation.
Conventional DAA circuits are of one of three types: transformer coupled, capacitive coupled, and optically coupled. Each of these types of DAA circuits has disadvantages.
Transformer coupled DAA circuits typically employ a discrete external transformer (i.e., a transformer provided as a discrete component that is separate from the integrated circuitry that is part of the DAA circuit and/or the CPE equipment as a whole). The external transformer provides signal coupling to the telephone line while also providing up to 1500 Vrms or greater of isolation from the CPE side (e.g., the modem side of the CPE; the CPE side of the equipment will also sometimes be referred to as the “home side” of the equipment, regardless of whether the CPE is installed in a residence or business). The transformer may provide good common mode rejection. However, the useful bandwidth of the transformer may be limited. For example, a transformer that is suitable for a voice-band modem may not be appropriate for a broadband DSL (digital subscriber line) modem (e.g., a modem for asymmetric DSL, symmetric DSL or very high capacity DSL). Conversely, a transformer that is suitable for a DSL modem may not be suitable for a voice-band modem. As a result, it may be difficult to provide a transformer coupled DAA for a modem to be used for both voice and data. Moreover, the external transformer may be large and expensive.
Capacitive coupled DAA circuits customarily use external high voltage capacitors for coupling to the telephone line with galvanic isolation. Up to three capacitors may be employed: one for the receive path, one for the transmit path, and one for the AC coupling return path. The capacitors may add significant cost to the DAA circuit, while also occupying a substantial amount of space on the circuit board.
Optically coupled DAA circuits may employ either external or integrated optical couplers. Integrated optical coupling may be accomplished by using multi-chip module packaging techniques to include the optical components in the IC package. Alternatively, externally located off-the-shelf opto-coupling ICs may be used. In either case, optical coupling may be significantly expensive. Furthermore, the optical couplers may be limited in performance and features.
In short, conventional DAA circuits add significant cost to CPE, while also often limiting the capabilities of the CPE.